The present invention relates generally to trunked radio communications systems, and in particular to a method and apparatus for trunked communication systems including home channel aliasing and call grouping.
Mobile radio communication systems provide for two way radio communications between a central control station and a fleet of cars, trucks, or other vehicles. Typical users of mobile radio communication systems include, for example, police departments, fire departments, taxi services and delivery services. Present mobile radio communication systems can be configured to provide for radio communications between the control station and all vehicles in a fleet, between the control station and selected vehicles in the fleet, or between different individual vehicles or groups of vehicles within a fleet.
Conventional mobile radio communication systems are typically organized with a number of vehicles and a control station assigned to a single common channel for a given coverage zone. A user assigned to the common channel must wait until no one else is transmitting on that channel before sending a message, because only one transmission at a time can be processed by a single channel. Even when a transmission is addressed to only one user in a conventional system (and therefore not heard by other users), the other users in the system must wait until that transmission is completed before they can use the system to communicate.
Mobile radio communication systems using transmission trunking are an improvement over conventional mobile radio communication systems in that trunked systems enable two or more users to communicate on the system at the same time. For instance, the dispatch console in a transmission trunked communication system can be communicating with one of the vehicles in the fleet; while, at the same time, two other vehicles in the fleet can be using the same trunked system to communicate with each other. Transmission trunked communication systems group a number of channels for the same coverage zone together into a single audio communication system, with each channel accessible to each user in the system. Because each user will only need to communicate over the trunked system part of the time, and because it is unlikely that all users will need to communicate at precisely the same time, the number of channels assigned to a trunked system group can always be less than the number of users allocated to that trunked system group.
The United States Federal Communications Commission (FCC) has assigned 600 channels in the 800 MHZ band for trunked land mobile use. Each channel is comprised of a pair of assigned frequencies, a mobile transmit frequency and a repeater transmit frequencies. Mobile transmit frequencies are 806-821 MHZ, with the repeater transmit frequencies exactly 45 MHZ above the corresponding mobile transmit frequency, or 851-866 MHZ. Channel spacing is 25 KHz, with the maximum allowed deviation between channels being xc2x15 KHz. In September 1987, the FCC also allocated 399 channels in the 900 MHZ band for trunked land mobile use. Mobile transmit frequencies are 896-901 MHZ, with the repeater transmit frequencies exactly 39 MHZ above the corresponding mobile transmit frequency, or 935-940 MHZ. Channel spacing is 12.5 KHz, with the maximum allowed deviation between channels being xc2x12.5 KHz.
In transmission trunked communication systems, a signaling protocol is used to send and receive control signals among users on each channel in the trunked system and a switching protocol is used to establish which channels those users will be communicating over. The preferred conventional transmission trunked communication system uses a signaling protocol that transmits the control signals in the subaudio band simultaneously with the transmission of voice or data information signals. Signaling protocols that can communicate control signals within the constraints of the subaudio band are preferred, because use of the subaudio band precludes the need for using a dedicated channel for transmitting the control signals (thereby reducing the number of available channels in the trunked system for voice and data communications). A switching protocol is used by the trunked system to automatically find and engage an open channel when a user initiates a transmission. To maximize the trunking capabilities of such a system, the switching protocol must efficiently allocate channels in the trunked system and avoid channels that are already in use at the time the transmission is initiated. For further explanation of a conventional transmission trunked communication system, reference is made to the description of the operation of the ClearChannel LTR(copyright) system contained in the manual entitled xe2x80x9cE. F. Johnson ClearChannel LTR Application Notexe2x80x9d, Part No. 009-0001-020 (Rev. 5, Oct. 1988), available from E. F. Johnson Company, Waseca, Minn., which is fully incorporated by reference herein. New transmission trunked communications systems have also been proposed which incorporate advances in network communications. For example, the E. F. Johnson MULTINET(trademark) system, part of which is described in U.S. Pat. No. 5,159,701 to Bames, et al., which is incorporated by reference in its entirety. U.S. Pat. No. 5,159,701 provides for, among other things, distributively interconnecting a plurality of land mobile trunked transmission communication systems into a wide area network.
There is a need in the art for a trunked radio communication system which supports large volumes of traffic and which provides efficient and reliable channel allocation.
A channel assignment system for a trunked radio communication system is provided to facilitate communications for a plurality of subscribers in the trunked radio communication system.
Trunked systems require the sharing of a relatively small number of channels with respect to the number of subscribers the system may support. The large volume of traffic results in collisions between subscribers attempting to use the same channels. This is particularly problematic where the system employs a xe2x80x9chome channelxe2x80x9d concept. A home channel is a system by which subscribers are assigned a particular channel upon which communications relating to that subscriber are initially conducted. One problem with the home channel concept is that a large volume of traffic can result in long delays between a subscriber""s request for a channel and receipt of a channel to conduct communications.
Additionally, a large volume of traffic can also result in a splitting of talk groups. A talk group is a collection of subscribers who receive like instructions for channels to use for reception and transmission in order to maintain consistency in communications for all subscribers in the talk group. For example, splitting of a talk group may occur if two or more subscribers of a same talk group request a channel at approximately the same time and some of the subscribers from the same talk group may be directed to conduct communications on different channels.
These and other issues are addressed by the various embodiments provided for by the present system. In one embodiment, the home channels of the present system are distributed to avoid excessive use of a single channel. In one embodiment, a home channel assignment system is provided to assign home channels to a subscriber in a trunked radio communication system. Home channel assignment is provided for the cases where the subscriber enters a locality including the preassigned home channel to the subscriber and where the locality does not include the preassigned home channel to the subscriber. In one embodiment, the home channel is assigned to the next lower available home channel if the preassigned home channel is not active in a particular locality. In one embodiment, the home channel is assigned to the next higher available home channel if the preassigned home channel is not active in a particular locality.
In one embodiment, a call grouping system is provided to avoid talk group splitting. Upon a channel request by a subscriber having its own home channel and talk group, a repeater checks other channels in a system to ensure that communications for a given home channel and talk group are not being conducted on another channel. In one embodiment a single check is performed for other channels conducting communications having the identical home channel and talk group. In another embodiment, a double check is available for other channels conducting communications having the identical home channel and talk group.
In one embodiment, both home channel aliasing and the call grouping are performed.
Other embodiments are provided in the detailed description, below, and the scope of the invention is to be determined by the scope of the appended claims and their equivalents.